It will be understood certain articles and components such as aerofoil blades for gas turbine engines are preferably formed with known crystallography in order to achieve desired operational performance. Thus, the components may be more resistant to high temperature creep or distortion. A number of methods have been identified for forming single crystal components and these generally utilise a seed crystal to initiate crystal forming in the component within a mould. What is generally required is a mechanism to ensure that the desired crystallography is achieved.
Each of these methods has disadvantages in attempting to successfully circumvent hazards with respect to stray grain nucleation in growth. For example: There is a change in the flux mode from chilled conduction to furnace radiation during initial withdrawal as a component is formed of up to 3 mm as the seed exits the hot zone of the melt of the molten material cast in order to form the component. This change in heat flux invariably leads to a thermal transient and it has been shown that initial withdrawal rates differing by a factor of 10 produced no appreciable difference in local heat transfer characteristics except where melt back is near to the base of the seed such that upon initial withdrawal there is a significant transient associated with the radial heat flux as the melt back exits the hot zone and melt back remote from the seed base where initial solidification in the seed is controlled by conduction through the solid but rapidly increases when radiation commences from the solid seed surface, i.e. when sufficient length of seed exists from the hot zone before settling to a steady-state value again but with radiation now controlled. In such circumstances, given that there are different heat transfer modes, it is not possible to altogether suppress the transient and additionally this leads to a lack of consistency in the process requiring the use of a spiral selector to mitigate the risk of a stray grain escaping into a forming component in a mould body. It will also be understood that the material from which the component forms can itself cause complications. Thus, with respect to nickel tungsten alloys where a visual inspection of the seed crystal prior to casting requires a chemical action, use of hydrofluoric acid does not meet acceptable Health and Safety Regulations for industrial usage. Finally, it will also be understood that it may be necessary to deliberately produce crystallographic orientations which are at an angle to the normal axial orientations and in these circumstances there will exist a wide range of axial orientations of stray grains that can grow competitively with the biased off-axial seed crystal preferred orientation and subsequently lead to orientation non-conformance.
It will be understood that typically the crystallography of the material from which an article or component is formed will generally be pre-known. Thus, for a crystal which has a face-centred cubic (fcc) structure there are three orthogonal growth directions (001, 010, 100) and there is four-fold symmetry. In such circumstances, there are a limited number of growth directions. In such circumstances, crystal growth in terms of dendritic growth during solidification will converge upon a wall of a mould or diverge from that wall in respective converging and diverging dispositions depending on axial orientation of seed. It is control of these misalignments of the crystallography which is necessary in order to create a suitable single crystal component or article.